Vector学习

分析

public class Vector<E>
    extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable

protected Object[] elementData;
protected int elementCount;
// 容量增量，可以指定在扩容时增加多少个位子
protected int capacityIncrement;

构造函数

public Vector() {
    this(10);
}

public Vector(int initialCapacity) {
    this(initialCapacity, 0);
}

public Vector(int initialCapacity, int capacityIncrement) {
    super();
    if (initialCapacity < 0)
        throw new IllegalArgumentException();
    this.elementData = new Object[initialCapacity];
    this.capacityIncrement = capacityIncrement;
}

public Vector(Collection<? extends E> c) {
    elementData = c.toArray();
    elementCount = elementData.length;
    if (elementData.getClass() != Object[].class)
        elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
}

常用方法

1、新增元素

public synchronized boolean add(E e) {
    modCount++;
    ensureCapacityHelper(elementCount + 1);
    elementData[elementCount++] = e;
    return true;
}

public synchronized void insertElementAt(E obj, int index) {
    modCount++;
    if (index > elementCount) {
        throw new ArrayIndexOutOfBoundsException();
    }
    ensureCapacityHelper(elementCount + 1);
    System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
    elementData[index] = obj;
    elementCount++;
}

public synchronized void addElement(E obj) {
    modCount++;
    ensureCapacityHelper(elementCount + 1);
    elementData[elementCount++] = obj;
}

2、扩容

private void ensureCapacityHelper(int minCapacity) {
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

private void grow(int minCapacity) {
    int oldCapacity = elementData.length;
    // 判断每次扩容位子，可由构造函数指定，默认一倍
    int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity);
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    elementData = Arrays.copyOf(elementData, newCapacity);
}

3、删除元素

public synchronized boolean removeElement(Object obj) {
    modCount++;
    int i = indexOf(obj);
    if (i >= 0) {
        removeElementAt(i);
        return true;
    }
    return false;
}

public synchronized void removeElementAt(int index) {
    modCount++;
    if (index >= elementCount) {
        throw new ArrayIndexOutOfBoundsException();
    }
    else if (index < 0) {
        throw new ArrayIndexOutOfBoundsException(index);
    }
    int j = elementCount - index - 1;
    if (j > 0) {
        System.arraycopy(elementData, index + 1, elementData, index, j);
    }
    elementCount--;
    elementData[elementCount] = null;
}

4、查找元素

public synchronized E get(int index) {
    if (index >= elementCount)
        throw new ArrayIndexOutOfBoundsException(index);
    return elementData(index);
}

public synchronized int indexOf(Object o, int index) {
    if (o == null) {
        for (int i = index ; i < elementCount ; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = index ; i < elementCount ; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

5、遍历

• 随机访问：Vector实现了RandmoAccess接口

for(int i = 0 ; i < vec.size() ; i++) { 
    value = vec.get(i);
}

• 迭代器

Iterator it = vec.iterator(); 
while(it.hasNext()){ 
    value = it.next();  
    // do something
}

• foreach

for(Integer value : vec) {
    //do something
}

• Enumeration

Vector vec = new Vector<>();    
Enumeration enu = vec.elements();    
while (enu.hasMoreElements()) {        
  	value = (Integer) enu.nextElement();    
}

小总结

Vector是否允许空	允许
Vector是否允许重复数据	允许
Vector是否有序	有序
Vector是否线程安全	线程安全